Gauss type lens system

ABSTRACT

The present invention provides an improved Gauss type lens system of a five unit six element construction which fulfills the following lens conditions: ##EQU1## wherein: f represents the focal length of the whole lens system; r 8  represents the radius of curvature of the intermediate cemented surface of the doublet of the fourth unit; Σd represents the total length of the whole lens system; f 1  represents the focal length of the first unit; f 45  represents the focal length of the fourth unit; and F 23  represents the focal length of an air lens formed between the second and third units (F 23  =N 3  /φ 23 , if the refractive power of the air lens is φ 23 ).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an objective lens system for use in asingle lens reflex camera, and more particularly, to an improved compactGauss type lens system of five units and six lens elements.

2. Description of the Prior Art

Numerous attempts have been made in the prior art to improve Gauss typelens systems for single lens reflex cameras. Recently, the demands ofthe consuming public have required smaller and more compact lightweightsingle reflex cameras. Attempts have been made to provide five lensunits, six lens elements systems of a compact configuration, such asU.S. Pat. Nos. 4,131,335 and 3,817,603. Additional examples are shown inJapanese Patent publication No. 38-11587, and Japanese Laid-Open Patentapplication Nos. 52-114322, 49-53419, 52-108426, 52- 146620, 53-110822,53-131034, 54-104334 and 54-156629.

As can be readily determined, the prior art is relatively crowded. Someof these prior art Gauss type lens systems are constructed of relativelyeconomical glass material of low refractive indices and provide arelatively long back focal length. There have been problems, however, inshortening the total length of the lens system to meet the requirementsof compactness and lightness in weight. Attempts to satisfy theseconflicting design parameters have been made with the use of arelatively expensive glass material of relatively high refractiveindices.

As can be readily appreciated, the rigors of competition in the cameraindustry forces a lens manufacturer to be acutely aware of the costcompetitiveness of his product. There is a demand in the photographyfield to provide an improved Gauss type lens system of a five lens unit,six lens element that provides an adequate back focal length whileshortening the total length of the lens system with the use of arelatively economical glass material while of course providing propercorrection of aberrations.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an improved Gauss typelens system of a five lens unit, six lens elements for use in a singlelens reflex camera which is both economical and compact.

Another object of the present invention is to provide a modified Gausstype lens system of a five unit, six lens elements having a sufficientback focal distance that is longer than about 72% of the focal length ofthe whole lens system, has a field angle of about 46° and an apertureratio of approximately 1/2.0.

A still another object of the present invention is to provide the abovementioned lens system with the distortion well corrected as well asother aberrations.

A further object of the present invention is to use relatively lowrefractive indexed glass in the manufacturing of the compact lenssystem.

The present invention is directed to an improved Gauss type lens systemof a five lens unit, six lens elements design having from the object tothe image side a positive meniscus single lens element convex to theobject side; a first positive meniscus single lens element conves to theobject side; a second positive meniscus single lens element convex tothe object side; a third negative meniscus single lens element convex tothe object side; a fourth doublet and a fifth positive lerns element.The lens system fulfills the following conditions: ##EQU2## wherein: frepresents the focal length of the whole lens system;

r₈ represents the radius of curvature of the intermediate cementedsurface of the doublet of the fourth unit;

Σd represents the total length of the whole lens system;

f₁ represents the focal length of the first unit;

f₄₅ represents the focal length of the fourth unit; and

F₂₃ represents the focal length of an air lens formed between the secondand third units (F₂₃ =N₃ /φ₂₃, if the refractive power of the air lensis φ₂₃).

The objects and features of the present invention whichare believed tobe novel are set forth with particularity in the appendant claims. Thepresent invention, both as to its organization and manner of operation,together with further objects and advantages thereof, may best beunderstood by reference to the following description, taken inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS:

FIG. 1 represents a schematic cross sectional view of a first embodimentof the present invention;

FIGS. 2a, 2b and 2c represent graphic plots of the aberrations of thefirst embodiment;

FIG. 3 represents a schematic cross sectional view of a secondembodiment of the present invention;

FIGS. 4a, 4b and 4c represent graphic plots of the aberrations of thesecond embodiment;

FIG. 5 represents a schematic cross sectional view of a third embodimentof the present invention;

FIGS. 6a, 6b and 6c represent graphic plots of the aberrations of thethird embodiment;

FIG. 7 represents a schematic cross sectional view of a fourthembodiment of the present invention;

FIGS. 8a, 8b and 8c represent graphic plots of the aberrations of thefourth embodiment;

FIG. 9 represents a schematic cross sectional view common to a fifth toeighth embodiments of the present invention;

FIGS. 10a, 10b and 10c represent graphic plots of the aberrations of thefifth embodiment;

FIGS. 11a, 11b and 11c represent graphic plots of the aberrations of thesixth embodiment.

FIGS. 12a, 12b and 12c represent graphic plots of the aberrations of theseventh embodiment; and

FIGS. 13a, 13b and 13c represent graphic plots of the aberrations of theeighth embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description is provided to enable any person skilled inthe optical art to make and use the present invention and sets forth thebest modes contemplated by the inventor of carrying out his invention.Various modifications, however, will remain readily apparent to thoseskilled in the art, since the generic principles of the presentinvention have been defined herein specifically to provide a compactimproved Gauss type lens system.

The derivation of the specific parameters of the lens embodimentsdisclosed herein can be accomplished with the assistance of a computer.The present invention represents the parameters of a compromised balanceof acceptable aberrations in a relatively easily manufactured lensassembly that is particularly suited for a single lens reflex camera.The data presented herein in the tables, to be discussed subsequently,are more than adequate to permit a competent artisan in the optical artto reproduce the embodiments of the present invention. In theaccompanying drawings, which supplement the following tables, the lensesin accordance with the present invention are illustrated schematically.As usual, in conventional lens diagrams, the light is assumed to travelfrom left to right and the individual lens units are designated by theletter, L, with a subscript indicating the position of the lenses asnumbered consecutively from the object to image side.

the radius of curvature is indicated by letter, r, with a subscriptcorresponding to the consecutive surfaces of the lens elements from leftto right. The axial spacings are designated with the letter, d, againwith a subscript indicating the distanceconsecutively from the left tothe right.

In the tables, the minus (-) signs indicate surfaces concave toward theobject side, while the surfaces without a sign are convex toward theobject side. The axial spacings will include both the thickness of thelens and the air spaces, and in this regard, the value of the axialspacings between the lens elements are appropriately positioned in thetables relative to the values of the radius of curvature to indicatewhether the axial distance is an air space or a thickness of the lens.The tables also provide, with respect to each embodiment, the fieldangle 2 ω,the F number, and the focal length. The refractive index isdesignated as N, while the Abbe number is V.

As shown in FIGS. 1, 3, 5, 7 and 9, the present invention provides animproved Gauss type lens system of five lens units, six lens elementscomprising from the object to the image side, a first group consistingof a positive meniscus single lens element (L₁) convex to the objectside; a second unit consisting of a positive meniscus single lenselement (L₂) convex to the object side; a third unit consisting of anegative meniscus single lens element (L₃) convex to the object side; afourth unit consisting of a doublet composed of a biconcave lens element(L₄) and a biconvex lens element (L₅) cemented to the biconcave lenselement at the image side thereof; and a fifth unit consisting of apositive lens element (L₆). The absolute value of the radius ofcurvature of the object side surface (r₁₀) of the positive lens elementbeing greater than that of the image side surface (r₁₁) of the positivelens element. The lens system fulfills the following conditions:##EQU3## wherein: f represents the focal length of the whole lenssystem;

r₈ represents the radius of curvature of the intermediate cementedsurface of the doublet of the fourth unit;

Σd represents the total length of the whole lens system;

f₁ represents the focal length of the first unit;

f₄₅ represents the focal length of the fourth unit; and

F₂₃ represents the focal length of an air lens formed between the secondand third units (F₂₃ =N₃ /φ₂₃, if the refractive power of the air lensif φ₂₃).

Condition (1) defines the radius of curvature of the intermediatecemented surface (r₈) of the doublet formed by elements L₄ and L₅, thecemented surface (r₈) being convex to the object side since the elementL₄ is biconcave and the element L₅ biconvex. When the cemented surface(r₈) is convex to the object side as in the present invention, the axialthickness (d₈) of the element L₅ normally increases to make the lenssystem bulky. According to the present invention, however, thecompactness of the lens system is maintained since condition (1) definesthe radius of curvature of the cemented surface (r₈) and the thickness(d₈) is decreased, with the appropriate aberration correction. Comaflare and longitudinal chromatic aberration would be generally createdand the lateral chromatic aberration would be in an over corrected stateif the lower limit of condition (1) is violated. On the contrary, if theupper limit of condition (1) is violated, the spherical aberration wouldbe under corrected and the field curvature would deteriorate.

Condition (2) provides a range for the total length of the whole lenssystem that is possible within the restriction of condition (1). If thelower limit of condition (2) is violated, any desirable aberrationcorrection would be impossible. On the other hand, compactness would notbe attainable if the upper limit of condition (2) is violated, althoughthe aberration correction may be easy.

Conditions (3), (4) and (5) are provided to obtain a sufficiently longback focal distance. In othe words, the refractive power of the firstunit is increased to decrease the necessary diameters of the second tofifth units, and, in response thereto, a relatively great negativerefractive power is distributed to the doublet of the fourth unit andthe air lens formed between the second and third units. Thus, asufficiently long back focal distance is secured. If the upper limit ofcondition (3) or the lower limits of condition (4) or )5) are violated,then the back focal distance would be undesirably shortened. On theother hand, the balance of aberration correction is broken due to anexcessively increased Petzval Sum, if the lower limit of condition (3)or the upper limits of condition (4) or (5) are violated.

For the compactness of the lens system, it is recommended to keep thefollowing conditions. ##EQU4## wherein, N₁, N₂ -N₆ represent refractiveindices of the glass material numbered from the object side,respectively.

Conditions (6) and (7) relate to the refractive indices of the glassmaterials used in the lens system of the present invention. Theseconditions insure a decrease in the spherical aberration and Petzval Sumto improve the field curvature, and for decreasing the coma flare. Whencondition (6) or (7) is violated,the compactness of the system would belost since it is inevitably necessary to increase the total length ofthe system for decreasing the Petzval Sum. If the condition (6) and (7)are kept, it is preferable to reduce conditions (3) and (4) as follows:##EQU5##

For the correction of the distortion with other aberrations kept in goodcondition, it is recommended to keep the following conditions: ##EQU6##wherein, r₆, r₇, r₁₀, and r₁₁ represent the radii of curvature of thesixth, seventh, tenth and eleventh surfaces numbered from the objectside, respectively; and v₃, v₄ represent Abbe numbers of the glassmaterial numbered from the object side, respectively. If the aboveconditions (8) to (14) are kept, it is preferable to reduce conditions(1) and (3) to (5) as follows: ##EQU7##

conditions (8) to (10) are for correcting distortion. If any one of theconditions (8) to (10) is violated, it is difficult to sufficientlycorrect distortion and some distortion would remain. For correctingdistortion only, a flattened tenth surface (r₁₀) and an increased radiusof curvature of the eleventh surface (r₁₁) as possible are generallyfavorable. However, these are limited for the purpose of total balanceof the correction of various aberrations. Thus, conditions (8) to (10)are introduced for correcting distortion with other aberrations alsocorrected. From other point of view, conditions (8) to (10) can bedetermined by considering Seidel coefficients V₆, V₇, V₁₀ and V₁₁ whichare coefficients of distortion with respect to sixth, seventh, tenth andeleventh surfaces (r₆, r₇, r₁₀ and r₁₁), respectively. In a case thatall aberrations are sufficiently corrected, a condition, V₆ +V₇ +V₁₀+V₁₁ ≦0.8is found to be favorable to reduce the negative distortion lessthan 1 percent in contrast to a reversed condition, V₆ +V₇ +V₁₀+V₁₁ >0.8, which frequently results in a negative distortion greaterthan 1 percent. The combination of conditions (8) to (10) are atranslation from the above favorable condition of Seidel coefficients interms of radii of curvature of the corresponding refractive surfaces.

Conditions (11) to (14) relates to the glass materials used in the lenssystem of the present invention. The lower limits of both conditions(11) and (12) are necessary to improve the field curvature by reducingthe spherical aberration and Petzval Sum ΣP, and also to reduce comaflare. On the other hand, the upper limits of both conditions (11) and(12) means that the present invention is characterized by the use ofglass materials of relatively low refractive indices. With respect toconditions (13) and (14), which are for correcting chromatic aberrationby means of the third and fourth units, the correction of thelongitudinal chromatic aberration would be excessive over the upperlimits and insufficient under the lower limits in both conditions (13)and (14).

The following Tables 1-8 represent, respectively, the eight embodimentsof the present invention.

                  TABLE 1                                                         ______________________________________                                        (Embodiment 1)                                                                f = 100  F.sub.No = 2.0  2ω = 46°  L.B = 72.06                   Radius of        Axial    Refractive                                                                              Abbe                                      Curvature        Distance Index     Number                                    ______________________________________                                        r.sub.1                                                                             51.34                                                                                        d.sub.1                                                                           7.23 N.sub.1 1.7290                                                                          ν.sub.1 53.5                       r.sub.2                                                                             253.55                                                                                       d.sub.2                                                                           0.30                                                 r.sub.3                                                                             35.71                                                                                        d.sub.3                                                                           6.33 N.sub.2 1.6584                                                                          ν.sub.2 50.9                       r.sub.4                                                                             51.20                                                                                        d.sub.4                                                                           2.59                                                 r.sub.5                                                                             105.32                                                                                       d.sub.5                                                                           2.37 N.sub.3 1.6727                                                                          ν.sub.3 32.2                       r.sub.6                                                                             26.43                                                                                        d.sub.6                                                                           18.56                                                r.sub.7                                                                             -31.21                                                                                       d.sub.7                                                                           2.13 N.sub.4 1.6166                                                                          ν.sub.4 36.7                       r.sub.8                                                                             80.00                                                                                        d.sub.8                                                                           8.30 N.sub.5 1.6783                                                                          ν.sub.5 49.0                       r.sub.9                                                                             -45.38                                                                                       d.sub.9                                                                           0.33                                                 r.sub.10                                                                            -1538.39                                                                                     d.sub.10                                                                          5.00 N.sub.6  1.7440                                                                         ν.sub.6 44.9                       r.sub.11                                                                            -65.85                                                                                       Σd                                                                          53.14                                                 ##STR1##                                                                                         ##STR2##                                                   ##STR3##                                                                                         ##STR4##                                                  ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        (Embodiment 2)                                                                f = 100  F.sub.No = 2.0  2ω = 46°  L.B = 72.01                   Radius of       Axial    Refractive Abbe                                      Curvature       Distance Index      Number                                    ______________________________________                                        r.sub.1                                                                             51.94                                                                                       d.sub.1                                                                           8.50 N.sub.1 1.7545                                                                           ν.sub.1 51.6                       r.sub.2                                                                             295.62                                                                                      d.sub.2                                                                           0.30                                                  r.sub.3                                                                             37.26                                                                                       d.sub.3                                                                           6.25 N.sub.2 1.6783                                                                           ν.sub.2 49.0                       r.sub.4                                                                             51.20                                                                                       d.sub.4                                                                           2.45                                                  r.sub.5                                                                             125.00                                                                                      d.sub.5                                                                           2.26 N.sub.3 1.6727                                                                           ν.sub.3 32.2                       r.sub.6                                                                             27.32                                                                                       d.sub.6                                                                           19.12                                                 r.sub.7                                                                             -31.50                                                                                      d.sub.7                                                                           2.07 N.sub.4 1.6166                                                                           ν.sub.4 36.7                       r.sub.8                                                                             113.83                                                                                      d.sub.8                                                                           9.65 N.sub.5 1.6583                                                                           ν.sub.5 58.5                       r.sub.9                                                                             -45.12                                                                                      d.sub.9                                                                           0.30                                                  r.sub.10                                                                            859.66                                                                                      d.sub.10                                                                          5.50 N.sub.6 1.7015                                                                           ν.sub.6 41.2                       r.sub.11                                                                            -67.04                                                                                      Σd                                                                          56.40                                                  ##STR5##                                                                                         ##STR6##                                                   ##STR7##                                                                                         ##STR8##                                                  ______________________________________                                    

                  TABLE 3                                                         ______________________________________                                        (Embodiment 3)                                                                f = 100  F.sub.No = 2.0  2ω = 46°  L.B = 72.21                   Radius of       Axial    Refractive Abbe                                      Curvature       Distance Index      Number                                    ______________________________________                                        r.sub.1                                                                             47.74                                                                                       d.sub.1                                                                           7.21 N.sub.1 1.7290                                                                           ν.sub.1 53.5                       r.sub.2                                                                             267.03                                                                                      d.sub.2                                                                           0.30                                                  r.sub.3                                                                             37.61                                                                                       d.sub.3                                                                           6.30 N.sub.2 1.6584                                                                           ν.sub.2 50.9                       r.sub.4                                                                             52.35                                                                                       d.sub.4                                                                           2.52                                                  r.sub.5                                                                             127.00                                                                                      d.sub.5                                                                           2.32 N.sub.3 1.6727                                                                           ν.sub.3 32.2                       r.sub.6                                                                             26.89                                                                                       d.sub.6                                                                           18.53                                                 r.sub.7                                                                             -30.03                                                                                      d.sub.7                                                                           2.12 N.sub.4 1.6259                                                                           ν.sub.4 35.7                       r.sub.8                                                                             1035.05                                                                                     d.sub.8                                                                           8.30 N.sub.5 1.6779                                                                           ν.sub.5 53.4                       r.sub.9                                                                             -45.87                                                                                      d.sub.9                                                                           0.31                                                  r.sub.10                                                                            8941.34                                                                                     d.sub.10                                                                          5.00 N.sub.6 1.7440                                                                           ν.sub.6 44.9                       r.sub.11                                                                            -60.75                                                                                      Σd                                                                          52.91                                                  ##STR9##                                                                            ##STR10##                                                               ##STR11##                                                                           ##STR12##                                                              ______________________________________                                    

                  TABLE 4                                                         ______________________________________                                        (Embodiment 4)                                                                f = 100  F.sub.No = 2.0  2ω = 46°  L.B = 72.00                   Radius of       Axial    Refractive Abbe                                      Curvature       Distance Index      Number                                    ______________________________________                                        r.sub.1                                                                             58.85                                                                                       d.sub.1                                                                           9.25 N.sub.1 .7495                                                                            ν.sub.1 50.1                       r.sub.2                                                                             251.49                                                                                      d.sub.2                                                                           0.30                                                  r.sub.3                                                                             39.22                                                                                       d.sub.3                                                                           6.36 N.sub.2 1.6710                                                                           ν.sub.2 51.8                       r.sub.4                                                                             65.06                                                                                       d.sub.4                                                                           3.74                                                  r.sub.5                                                                             101.79                                                                                      d.sub.5                                                                           2.56 N.sub.3 1.6727                                                                           ν.sub.3 32.2                       r.sub.6                                                                             26.07                                                                                       d.sub.6                                                                           22.80                                                 r.sub.7                                                                             -29.60                                                                                      d.sub.7                                                                           2.04 N.sub.4 1.6057                                                                           ν.sub.4 37.8                       r.sub.8                                                                             75.16                                                                                       d.sub.8                                                                           11.19                                                                              N.sub.5 1.6583                                                                           ν.sub.5 58.5                       r.sub.9                                                                             -40.55                                                                                      d.sub.9                                                                           0.30                                                  r.sub.10                                                                            3667.44                                                                                     d.sub.10                                                                          6.43 N.sub.6 1.7352                                                                           ν.sub.6 40.9                       r.sub.11                                                                            -71.02                                                                                      Σd                                                                          64.97                                                  ##STR13##                                                                           ##STR14##                                                               ##STR15##                                                                           ##STR16##                                                              ______________________________________                                    

                  TABLE 5                                                         ______________________________________                                        (Embodiment 5)                                                                f = 100  F.sub.No = 2.0  2ω = 46°  L.B. = 72.01                  Radius of       Axial    Refractive Abbe                                      Curvature       Distance Index      Number                                    ______________________________________                                        r.sub.1                                                                             73.73                                                                                       d.sub.1                                                                           8.03 N.sub.1 1.7200                                                                           ν.sub.1 50.3                       r.sub.2                                                                             560.82                                                                                      d.sub.2                                                                           0.32                                                  r.sub.3                                                                             39.32                                                                                       d.sub.3                                                                           8.59 N.sub.2 1.7200                                                                           ν.sub.2 50.3                       r.sub.4                                                                             66.67                                                                                       d.sub.4                                                                           1.90                                                  r.sub.5                                                                             109.93                                                                                      d.sub.5                                                                           3.06 N.sub.3 1.6830                                                                           ν.sub.3 32.1                       r.sub.6                                                                             28.48                                                                                       d.sub.6                                                                           22.71                                                 r.sub.7                                                                             -31.97                                                                                      d.sub.7                                                                           4.36 N.sub.4 1.6545                                                                           ν.sub.4 33.9                       r.sub.8                                                                             326.53                                                                                      d.sub.8                                                                           8.80 N.sub.5 1.7200                                                                           ν.sub.5 52.1                       r.sub.9                                                                             -42.70                                                                                      d.sub.9                                                                           0.32                                                  r.sub.10                                                                            ∞                                                                                     d.sub.10                                                                          5.71 N.sub.6 1.7440                                                                           ν.sub.6 44.9                       r.sub.11                                                                            -76.60                                                                                      Σd                                                                          63.80                                                  ##STR17##                                                                           ##STR18##                                                               ##STR19##                                                                           ##STR20##                                                              ______________________________________                                    

                  TABLE 6                                                         ______________________________________                                        (Embodiment 6)                                                                f = 100  F.sub.No = 2.0  2ω = 46°  L.B. = 71.90                  Radius of       Axial    Refractive Abbe                                      Curvature       Distance Index      Number                                    ______________________________________                                        r.sub.1                                                                             73.07                                                                                       d.sub.1                                                                           7.98 N.sub.1 1.7200                                                                           ν.sub.1 50.3                       r.sub.2                                                                             557.50                                                                                      d.sub.2                                                                           0.31                                                  r.sub.3                                                                             39.19                                                                                       d.sub.3                                                                           8.70 N.sub.2 1.7200                                                                           ν.sub.2 50.3                       r.sub.4                                                                             65.35                                                                                       d.sub.4                                                                           1.75                                                  r.sub.5                                                                             109.30                                                                                      d.sub.5                                                                           3.09 N.sub.3 1.6830                                                                           ν.sub.3 32.1                       r.sub.6                                                                             28.45                                                                                       d.sub.6                                                                           22.64                                                 r.sub.7                                                                             -31.83                                                                                      d.sub.7                                                                           4.17 N.sub.4 1.6545                                                                           ν.sub.4 33.9                       r.sub.8                                                                             316.75                                                                                      d.sub.8                                                                           8.76 N.sub.5 1.7200                                                                           ν.sub.5 52.1                       r.sub.9                                                                             -42.29                                                                                      d.sub.9                                                                           0.31                                                  r.sub.10                                                                            ∞                                                                                     d.sub.10                                                                          5.69 N.sub.6 1.7440                                                                           ν.sub.6 44.9                       r.sub.11                                                                            -77.02                                                                                      Σd                                                                          63.40                                                  ##STR21##                                                                           ##STR22##                                                               ##STR23##                                                                           ##STR24##                                                              ______________________________________                                    

                  TABLE 7                                                         ______________________________________                                        (Embodiment 7)                                                                f = 100  F.sub.No = 2.0  2ω = 46°  L.B. = 71.96                  Radius of       Axial    Refractive Abbe                                      Curvature       Distance Index      Number                                    ______________________________________                                        r.sub.1                                                                             73.11                                                                                       d.sub.1                                                                           7.99 N.sub.1 1.7200                                                                           ν.sub.1 50.3                       r.sub.2                                                                             551.78                                                                                      d.sub.2                                                                           0.31                                                  r.sub.3                                                                             39.19                                                                                       d.sub.3                                                                           8.71 N.sub.2 1.7200                                                                           ν.sub.2 50.3                       r.sub.4                                                                             65.38                                                                                       d.sub.4                                                                           1.75                                                  r.sub.5                                                                             108.94                                                                                      d.sub.5                                                                           3.08 N.sub.3 1.6830                                                                           ν.sub.3 32.1                       r.sub.6                                                                             28.44                                                                                       d.sub.6                                                                           22.63                                                 r.sub.7                                                                             -31.84                                                                                      d.sub.7                                                                           4.18 N.sub.4 1.6545                                                                           ν.sub.4 33.9                       r.sub.8                                                                             316.62                                                                                      d.sub.8                                                                           8.76 N.sub.5 1.7200                                                                           ν.sub.5 52.1                       r.sub.9                                                                             -42.36                                                                                      d.sub.9                                                                           0.31                                                  r.sub.10                                                                            ∞                                                                                     d.sub.10                                                                          5.69 N.sub. 6 1.7440                                                                          ν.sub.6 44.9                       r.sub.11                                                                            -76.86                                                                                      Σd                                                                          63.41                                                  ##STR25##                                                                           ##STR26##                                                               ##STR27##                                                                           ##STR28##                                                              ______________________________________                                    

                  TABLE 8                                                         ______________________________________                                        (Embodiment 8)                                                                f = 100  F.sub.No = 2.0  2ω = 46°  L.B. = 72.04                  Radius of       Axial    Refractive Abbe                                      Curvature       Distance Index      Number                                    ______________________________________                                        r.sub.1                                                                             68.36                                                                                       d.sub.1                                                                           8.00 N.sub.1 1.7200                                                                           ν.sub.1 50.3                       r.sub.2                                                                             447.66                                                                                      d.sub.2                                                                           0.31                                                  r.sub.3                                                                             39.60                                                                                       d.sub.3                                                                           8.60 N.sub.2 1.7200                                                                           ν.sub.2 50.3                       r.sub.4                                                                             64.10                                                                                       d.sub.4                                                                           1.75                                                  r.sub.5                                                                             105.17                                                                                      d.sub.5                                                                           3.11 N.sub.3 1.6830                                                                           ν.sub.3 32.1                       r.sub.6                                                                             28.13                                                                                       d.sub.6                                                                           22.34                                                 r.sub.7                                                                             -31.44                                                                                      d.sub.7                                                                           4.33 N.sub.4 1.6545                                                                           ν.sub.4 33.9                       r.sub.8                                                                             309.37                                                                                      d.sub.8                                                                           8.82 N.sub.5 1.6935                                                                           ν.sub.5 53.4                       r.sub.9                                                                             -42.82                                                                                      d.sub.9                                                                           0.31                                                  r.sub.10                                                                            ∞                                                                                     d.sub.10                                                                          5.71 N.sub.6 1.7440                                                                           ν.sub.6 44.9                       r.sub.11                                                                            -69.83                                                                                      Σd                                                                          63.28                                                  ##STR29##                                                                           ##STR30##                                                               ##STR31##                                                                           ##STR32##                                                              ______________________________________                                    

FIGS. 2a, 2b and 2c are graphical plots of spherical aberration,astigmatism and distortion of the first embodiment of FIG. 1; FIGS. 4a,4b and 4c disclose the aberration plots for FIG. 3. FIGS. 6a, 6b and 6cdisclose the aberration plots for FIG. 5 while FIGS. 8a, 8b and 8cdisclose the aberration plots for FIG. 7. FIGS. 10a, 10b and 10c; 11a,11b and 11c; 12a, 12b and 12c; and 13a, 13b and 13c disclose theaberration plots for the fifth to eighth embodiments which are commonlyshown in FIG. 9.

While the above embodiments have been disclosed as the best modespresently contemplated by the inventor, it should be realized that theseexamples should not be interpreted as limiting, because artisans skilledin this field, once given the present teachings, can vary from thesespecific embodiments. Accordingly, the scope of the present inventionshould be determined from the following claims,

What is claimed is:
 1. A modified Gauss type lens system of five unitssix-elements comprising from the object to the image side:a first unitconsisting of a positive meniscus single lens element convex to theobject side; a second unit consisting of a positive meniscus single lenselement convex to the object side; a third unit consisting of a negativemeniscus single lens element convex to the object side; a fourth unitconsisting of a doublet composed of a biconcave lens element and abiconvex lens element cemented to the biconcave lens element at theimage side thereof; and a fifth unit consisting of a positive lenselement, the absolute value of the radius of curvature of the objectside surface of the positive lens element being greater than that of theimage side surface of the positive lens element, wherein the lens systemfulfills the following conditions: ##EQU8## wherein f represents thefocal length of the whole lens system; r₈ represents the radius ofcurvature of the intermediate cemented surface of the doublet of thefourth unit; Σd represents the total length of the whole lens system; f₁represents the focal length of the first unit; f₄₅ represents the focallength of the fourth unit; F₂₃ represents the focal length of an airlens formed between the second and third units (F₂₃ =N₃ /φ₂₃, if therefractive power of the air lens is φ₂₃); and N₁ represents therefractive indices of the glass materials numbered consecutively fromthe object side.
 2. A modified Gauss type lens system of five-units,six-elements comprising from the object to the image side:a first unitconsisting of a positive meniscus single lens element convex to theobject side; a second unit consisting of a positive meniscus single lenselement convex to the object side; a thid unit consisting of a negativemeniscus single lens element convex to the object side; a fourth unitconsisting of a doublet composed of a biconcave lens element and abiconvex lens element cemented to the biconcave lens element at theimage side thereof; and a fifth unit consisting of a positive lenselement, the absolute value of the radius of curvature of the objectside surface of the positive lens element being greater than that of theimage side surface of the positive lens element, wherein the lens systemfulfills the following conditions: ##EQU9## wherein: f represents thefocal length of the whole lens system; r₆, r₇, r₈, r₁₀ and r₁₁ representradii of curvature of the sixth, seventh, eighth, tenth and eleventhsurfaces numbered from the object side, respectively; f₁ represents thefocal length of the first unit; f₄₅ represents the focal length of thefourth unit; F₂₃ represents the focal length of an air lens formedbetween the second and third units (F₂₃ =N₃ /φ₂₃, if the refractivepower of the air lens is φ₂₃); N₁, N₂. . . , N₆ represent refractiveindices of the glass materials numbered from the object side,respectively; and ν.sub.
 3. ν₄ represent Abbe numbers of the glassmaterials numbered from the object side, respectively.
 3. A modifiedGauss type lens system of a five-units, six-elements type comprisingfrom the object to the image side:a first unit consisting of a positivemeniscus single lens element convex to the object side; a second unitconsisting of a positive meniscus single lens element convex to theobject side; a third unit consisting of a negative meniscus single lenselement convex to the object side; a fourth unit consisting of a doubletcomposed of a biconcave lens element and a biconvex lens elementcemented to the biconcave lens element at the image side thereof, and afifth unit consisting of a positive lens element, the absolute value ofthe radius of curvature of the object side surface of the positive lenselement being greater than that of the image side surface of thepositive lens element, wherein the lens system fulfills the followingconditions: ##EQU10## wherein: f represents the focal length of thewhole lens system; f₈ represents the radius of curvature of theintermediate cemented surface of the doublet of the fourth unit; drepresents the total length of the whole lens system, and f₁ representsthe focal length of the first unit.
 4. A modified Gauss type lens systemof a five-units six-elements type comprising from the object to theimage side:a first unit consisting of a positive meniscus single lenselement convex to the object side; a second unit consisting of apositive meniscus single lens element convex to the object side; a thirdunit consisting of a negative meniscus single lens element convex to theobject side; a fourth unit of a negative refractive power consisting ofa doublet composed of a biconcave lens element and a biconvex lenselement cemented to the biconcave lens element at the image sidethereof, and a fifth unit consisting of a positive lens element, theabsolute value of the radius of curvature of the object side surface ofthe positive lens element being greater than that of the image sidesurface of the positive lens element, wherein the lens system fulfillsthe following conditions: ##EQU11## wherein: f represents the focallength of the whole lens system; f₄₅ represents the focal length of thefourth unit; r₆, r₇ and r₁₀ represent radii of curvature of the sixth,seventh, and tenth surfaces numbered from the object side, respectively,and N₁ represents the refractive index of the glass material of thefirst unit.
 5. The invention of claim 4, wherein the intermediatecemented surface of the doublet in the fourth unit has a positiverefractive power.
 6. A modified Gauss type lens system comprising thefollowing design parameters:

    ______________________________________                                        f = 100   F.sub.No = 2.0  2ω = 46°  L. B = 72.06                 Radius of    Axial      Refractive Abbe                                       Curvature    Distance   Index      Number                                     ______________________________________                                        r.sub.1                                                                             51.34                                                                                    d.sub.1                                                                              7.23  N.sub.1 1.7290                                                                         ν.sub.1 53.5                        r.sub.2                                                                             253.55                                                                                   d.sub.2                                                                              0.30                                                  r.sub.3                                                                             35.71                                                                                    d.sub.3                                                                              6.33  N.sub.2 1.6584                                                                         ν.sub.2 50.9                        r.sub.4                                                                             51.20                                                                                    d.sub.4                                                                              2.59                                                  r.sub.5                                                                             105.32                                                                                   d.sub.5                                                                              2.37  N.sub.3 1.6727                                                                         ν.sub.3 32.2                        r.sub.6                                                                             26.43                                                                                    d.sub.6                                                                              18.56                                                 r.sub.7                                                                             -31.21                                                                                   d.sub.7                                                                              2.13  N.sub.4 1.6166                                                                         ν.sub.4 36.7                        r.sub.8                                                                             80.00                                                                                    d.sub.8                                                                              8.30  N.sub.5 1.6783                                                                         ν.sub.5 49.0                        r.sub.9                                                                             -45.38                                                                                   d.sub.9                                                                              0.33                                                  .sub. r.sub.10                                                                      -1538.39                                                                                 .sub. d.sub.10                                                                       5.00  N.sub.6 1.7440                                                                         ν.sub.6 44.9                        .sub. -65.85.11                                                                                Σd                                                                             53.14                                                  ##STR33##                                                                                        ##STR34##                                                  ##STR35##                                                                                        ##STR36##                                                 ______________________________________                                    


7. A modified Gauss type lens system comprising the following designparameters:

    ______________________________________                                        f = 100  F.sub.No = 2.0  2ω = 46°  L. B. = 72.01                 Radius of    Axial      Refractive Abbe                                       Curvature    Distance   Index      Number                                     ______________________________________                                        r.sub.1                                                                             51.94                                                                                    d.sub.1                                                                              8.50  N.sub.1 1.7545                                                                         ν.sub.1 51.6                        r.sub.2                                                                             295.62                                                                                   d.sub.2                                                                              0.30                                                  r.sub.3                                                                             37.26                                                                                    d.sub.3                                                                              6.25  N.sub.2 1.6783                                                                         ν.sub.2 49.0                        r.sub.4                                                                             51.20                                                                                    d.sub.4                                                                              2.45                                                  r.sub.5                                                                             125.00                                                                                   d.sub.5                                                                              2.26  N.sub.3 1.6727                                                                         ν.sub.3 32.2                        r.sub.6                                                                             27.32                                                                                    d.sub.6                                                                              19.12                                                 r.sub.7                                                                             -31.50                                                                                   d.sub.7                                                                              2.07  N.sub.4 1.6166                                                                         ν.sub.4 36.7                        r.sub.8                                                                             113.83                                                                                   d.sub.8                                                                              9.65  N.sub.5 1.6583                                                                         ν.sub.5 58.5                        r.sub.9                                                                             -45.12                                                                                   d.sub.9                                                                              0.30                                                  .sub. r.sub.10                                                                      859.66                                                                                   .sub. d.sub.10                                                                       5.50  N.sub.6 1.7015                                                                         ν.sub.6 41.2                        .sub. r.sub. 11                                                                     -67.04                                                                                   Σd                                                                             56.40                                                  ##STR37##                                                                                        ##STR38##                                                  ##STR39##                                                                                        ##STR40##                                                 ______________________________________                                    


8. A modified Gauss type lens system comprising the following designparameters:

    ______________________________________                                        f = 100  F.sub.No = 2.0  2ω = 46°  L. B. = 72.21                 Radius of    Axial      Refractive Abbe                                       Curvature    Distance   Index      Number                                     ______________________________________                                        r.sub.1                                                                             47.74                                                                                    d.sub.1                                                                              7.21  N.sub.1 1.7290                                                                         ν.sub.1 53.5                        r.sub.2                                                                             267.03                                                                                   d.sub.2                                                                              0.30                                                  r.sub.3                                                                             37.61                                                                                    d.sub.3                                                                              6.30  N.sub.2 1.6584                                                                         ν.sub.2 50.9                        r.sub.4                                                                             52.35                                                                                    d.sub.4                                                                              2.52                                                  r.sub.5                                                                             127.00                                                                                   d.sub.5                                                                              2.32  N.sub.3 1.6727                                                                         ν.sub.3 32.2                        r.sub.6                                                                             26.89                                                                                    d.sub.6                                                                              18.53                                                 r.sub.7                                                                             -30.03                                                                                   d.sub.7                                                                              2.12  N.sub.4 1.6259                                                                         ν.sub.4 35.7                        r.sub.8                                                                             1035.05                                                                                  d.sub.8                                                                              8.30  N.sub.5 1.6779                                                                         ν.sub.5 53.4                        r.sub.9                                                                             -45.87                                                                                   d.sub.9                                                                              0.31                                                  .sub. r.sub.10                                                                      8941.34                                                                                  .sub. d.sub.10                                                                       5.00  N.sub.6 1.7440                                                                         ν.sub.6 44.9                        .sub. r.sub.11                                                                      -60.75                                                                                   Σd                                                                             52.91                                                  ##STR41##                                                                                        ##STR42##                                                  ##STR43##                                                                                        ##STR44##                                                 ______________________________________                                    


9. A modified Gauss type lens system comprising the following designparameters:

    ______________________________________                                        f = 100  F.sub.No = 2.0  2ω = 46°  L. B. = 72.00                 Radius of    Axial      Refractive Abbe                                       Curvature    Distance   Index      Number                                     ______________________________________                                        r.sub.1                                                                             58.85                                                                                    d.sub.1                                                                              9.25  N.sub.1 1.7495                                                                         ν.sub.1 50.1                        r.sub.2                                                                             251.49                                                                                   d.sub.2                                                                              0.30                                                  r.sub.3                                                                             39.22                                                                                    d.sub.3                                                                              6.36  N.sub.2 1.6710                                                                         ν.sub.2 51.8                        r.sub.4                                                                             65.06                                                                                    d.sub.4                                                                              3.74                                                  r.sub.5                                                                             101.79                                                                                   d.sub.5                                                                              2.56  N.sub.3 1.6727                                                                         ν.sub.3 32.2                        r.sub.6                                                                             26.07                                                                                    d.sub.6                                                                              22.80                                                 r.sub.7                                                                             -29.60                                                                                   d.sub.7                                                                              2.04  N.sub.4 1.6057                                                                         ν.sub.4 37.8                        r.sub.8                                                                             75.16                                                                                    d.sub.8                                                                              11.19 N.sub.5 1.6583                                                                         ν.sub.5 58.5                        r.sub.9                                                                             -40.55                                                                                   d.sub.9                                                                              0.30                                                  .sub. r.sub.10                                                                      3667.44                                                                                  .sub. d.sub.10                                                                       6.43  N.sub.6 1.7352                                                                         ν.sub.6 40.9                        .sub. r.sub.11                                                                      -71.02                                                                                   Σd                                                                             64.97                                                  ##STR45##                                                                                        ##STR46##                                                  ##STR47##                                                                                        ##STR48##                                                 ______________________________________                                    


10. A modified Gauss type lens system comprising the following designparameters:

    ______________________________________                                        f = 100  F.sub.No = 2.0  2ω = 46°  L. B. = 72.01                 Radius of    Axial      Refractive Abbe                                       Curvature    Distance   Index      Number                                     ______________________________________                                        r.sub.1                                                                             73.73                                                                                    d.sub.1                                                                              8.03  N.sub.1 1.7200                                                                         ν.sub.1 50.3                        r.sub.2                                                                             560.82                                                                                   d.sub.2                                                                              0.32                                                  r.sub.3                                                                             39.32                                                                                    d.sub.3                                                                              8.59  N.sub.2 1.7200                                                                         ν.sub.2 50.3                        r.sub.4                                                                             66.67                                                                                    d.sub.4                                                                              1.90                                                  r.sub.5                                                                             109.93                                                                                   d.sub.5                                                                              3.06  N.sub.3 1.6830                                                                         ν.sub.3 32.1                        r.sub.6                                                                             28.48                                                                                    d.sub.6                                                                              22.71                                                 r.sub.7                                                                             -31.97                                                                                   d.sub.7                                                                              4.36  N.sub.4 1.6545                                                                         ν.sub.4 33.9                        r.sub.8                                                                             326.53                                                                                   d.sub.8                                                                              8.80  N.sub.5 1.7200                                                                         ν.sub.5 52.1                        r.sub.9                                                                             -42.70                                                                                   d.sub.9                                                                              0.32                                                  .sub. r.sub.10                                                                      ∞                                                                                  .sub. d.sub.10                                                                       5.71  N.sub.6 1.7440                                                                         ν.sub.6 44.9                        .sub. r.sub. 11                                                                     -76.60                                                                                   Σd                                                                             63.80                                                  ##STR49##                                                                                        ##STR50##                                                  ##STR51##                                                                                        ##STR52##                                                 ______________________________________                                    


11. A modified Gauss type lens system comprising the following designparameters:

    ______________________________________                                        f = 100  F.sub.No = 2.0  2ω = 46°  L. B. = 71.90                 Radius of    Axial      Refractive Abbe                                       Curvature    Distance   Index      Number                                     ______________________________________                                        r.sub.1                                                                             73.07                                                                                    d.sub.1                                                                              7.98  N.sub.1 1.7200                                                                         ν.sub.1 50.3                        r.sub.2                                                                             557.50                                                                                   d.sub.2                                                                              0.31                                                  r.sub.3                                                                             39.19                                                                                    d.sub.3                                                                              8.70  N.sub.2 1.7200                                                                         ν.sub.2 50.3                        r.sub.4                                                                             65.35                                                                                    d.sub.4                                                                              1.75                                                  r.sub.5                                                                             109.30                                                                                   d.sub.5                                                                              3.09  N.sub.3 1.6830                                                                         ν.sub.3 32.1                        r.sub.6                                                                             28.45                                                                                    d.sub.6                                                                              22.64                                                 r.sub.7                                                                             -31.83                                                                                   d.sub.7                                                                              4.17  N.sub.4 1.6545                                                                         ν.sub.4 33.9                        r.sub.8                                                                             316.75                                                                                   d.sub.8                                                                              8.76  N.sub.5 1.7200                                                                         ν.sub.5 52.1                        r.sub.9                                                                             -42.29                                                                                   d.sub.9                                                                              0.31                                                  .sub. r.sub.10                                                                      ∞                                                                                  .sub. d.sub.10                                                                       5.69  N.sub.6 1.7440                                                                         ν.sub.6 44.9                        .sub. r.sub. 11                                                                     -77.02                                                                                   Σd                                                                             63.40                                                  ##STR53##                                                                                        ##STR54##                                                  ##STR55##                                                                                        ##STR56##                                                 ______________________________________                                    


12. A modified Gauss type lens system comprising the following designparameters:

    ______________________________________                                        f = 100  F.sub.No = 2.0  2ω = 46°  L. B. = 71.96                 Radius of    Axial      Refractive Abbe                                       Curvature    Distance   Index      Number                                     ______________________________________                                        r.sub.1                                                                             73.11                                                                                    d.sub.1                                                                              7.99  N.sub.1 1.7200                                                                         ν.sub.1 50.3                        r.sub.2                                                                             551.78                                                                                   d.sub.2                                                                              0.31                                                  r.sub.3                                                                             39.19                                                                                    d.sub.3                                                                              8.71  N.sub.2 1.7200                                                                         ν.sub.2 50.3                        r.sub.4                                                                             65.38                                                                                    d.sub.4                                                                              1.75                                                  r.sub.5                                                                             108.94                                                                                   d.sub.5                                                                              3.08  N.sub.3 1.6830                                                                         ν.sub.3 32.1                        r.sub.6                                                                             28.44                                                                                    d.sub.6                                                                              22.63                                                 r.sub.7                                                                             -31.84                                                                                   d.sub.7                                                                              4.18  N.sub.4 1.6545                                                                         ν.sub.4 33.9                        r.sub.8                                                                             316.62                                                                                   d.sub.8                                                                              8.76  N.sub.5 1.7200                                                                         ν.sub.5 52.1                        r.sub.9                                                                             -42.36                                                                                   d.sub.9                                                                              0.31                                                  .sub. r.sub.10                                                                      ∞                                                                                  .sub. d.sub.10                                                                       5.69  N.sub.6 1.7440                                                                         ν.sub.6 44.9                        .sub. r.sub. 11                                                                     -76.86                                                                                   Σd                                                                             63.41                                                  ##STR57##                                                                                        ##STR58##                                                  ##STR59##                                                                                        ##STR60##                                                 ______________________________________                                    


13. A modified Gauss type lens system comprising the following designparameters:

    ______________________________________                                        f = 100  F.sub.No = 2.0  2ω = 46°  L. B. = 72.04                 Radius of    Axial      Refractive Abbe                                       Curvature    Distance   Index      Number                                     ______________________________________                                        r.sub.1                                                                             68.36                                                                                    d.sub.1                                                                              8.00  N.sub.1 1.7200                                                                         ν.sub.1 50.3                        r.sub.2                                                                             447.66                                                                                   d.sub.2                                                                              0.31                                                  r.sub.3                                                                             39.60                                                                                    d.sub.3                                                                              8.60  N.sub.2 1.7200                                                                         ν.sub.2 50.3                        r.sub.4                                                                             64.10                                                                                    d.sub.4                                                                              1.75                                                  r.sub.5                                                                             105.17                                                                                   d.sub.5                                                                              3.11  N.sub.3 1.6830                                                                         ν.sub.3 32.1                        r.sub.6                                                                             28.13                                                                                    d.sub.6                                                                              22.34                                                 r.sub.7                                                                             -31.44                                                                                   d.sub.7                                                                              4.33  N.sub.4 1.6545                                                                         ν.sub.4 33.9                        r.sub.8                                                                             309.37                                                                                   d.sub.8                                                                              8.82  N.sub.5 1.6935                                                                         ν.sub.5 53.4                        r.sub.9                                                                             -42.82                                                                                   d.sub.9                                                                              0.31                                                  .sub. r.sub.10                                                                      ∞                                                                                  .sub. d.sub.10                                                                       5.71  N.sub.6 1.7440                                                                         ν.sub.6 44.9                        .sub. r.sub. 11                                                                     -69.83                                                                                   Σd                                                                             63.28                                                  ##STR61##                                                                                        ##STR62##                                                  ##STR63##                                                                                        ##STR64##                                                 ______________________________________                                    